Index-fund investors, known to take a hands-off approach to their money, are now being asked to make some big decisions -- and possibly take on more risk.

Index funds traditionally have aimed to mirror the performance of broad-market benchmarks, such as the Standard & Poor's 500-stock index or the Dow Jones Wilshire 5000 Composite index. This made the funds relatively predictable, since investors could expect to closely track the market's return, minus a small slice for management expenses. Another attraction: Index funds have historically outperformed the majority of funds that rely on a fund ...

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